Apparatus and methods for measuring magnetic fields and electric currents

ABSTRACT

Apparatus for sensing or measuring a magnetic field comprises a light source (10) for generating light having at least a component which is polarized, a sensing element (16) arranged to transmit light from the source and comprising material having a substantial Verdet constant which depends on the temperature of the material, means (14-19, 21) for combining light from the source with light from the sensing element to form an interference pattern, means (23) for providing a signal representative of the intensity of the light so combined, and means for deriving first and second signals dependent on the Verdet constant of the material and the temperature thereof, respectively, from the signal representative of light intensity, whereby the intensity of the magnetic field at the element, substantially independent of temperature, can be derived. The invention extends to a method of sensing or measuring magnetic field intensity, and to apparatus and a method for sensing or measuring electric current.

BACKGROUND OF THE INVENTION

The present invention relates to apparatus and a method for sensing ormeasuring magnetic fields and hence also to apparatus and a method forsensing or measuring electric current. Temperature may also be measured.A particular application is in measuring high fault currents inelectrical power and distribution systems. The invention could, forexample, be used in mapping magnetic fields around high powertransmitters or for condition monitoring in electrical machinery.

Considerable research has been carried out into the use of the Faradayeffect, which is described below, in measuring large electricalcurrents. Measurement depends on the Verdet constant V(λ,T) which isboth dispersive and temperature dependent. Most techniques haveconcentrated on single mode optical fiber as the sensing element becauseof its well-known desirable properties; electrical isolation,flexibility, linear response, large bandwidth and zero hysteresis.However, optical fibers still suffer from several serious problems.Firstly, environmental birefringence perturbations, the most serious ofwhich arises from vibration, which causes changes in the state ofpolarization (SOP) of the optical beam with concomitant sensitivity andscale factor fluctuations. Various techniques have been investigated toreduce this effect including the use of very low intrinsic birefringentfiber, the development of spun high birefringence fiber, the use ofjelly filled cabling to mechanically isolate the optical fiber and thedeployment of low birefringence fiber in a helical configuration to takeadvantage of geometrical birefringence effects. These techniques havemet with varying degrees of success, but all suffer from varyingproblems. A major problem is the low Verdet constant of silica fibers(˜5×10⁻⁶ rad A⁻¹ ) which then requires many turns of fiber to increasethe sensitivity. However, this then leads to greater birefringence inthe fiber introduced when the fiber is deployed in a loop around thecurrent carrying conductor, and hence larger diameter loops arerequired; the birefringence is proportional to (1/R²) where R is theloop radius. In addition this technique cannot be used for small sensingelements, for example localized sensors for mapping magnetic fieldsaround high power transmitters or for condition monitoring in electricalmachinery. Optical fiber has recently been employed as the sensingelement in constructing an electric current sensor for fault protection,up to 60 KA, on a 145 KV power line. However, the loop diameter is ˜1 mand up to 7 turns of fiber are required to give the required dynamicrange. This technique is inappropriate for the smaller sensingconfigurations mentioned previously, and where lower currents, 1 to1000A, are required; the reduction in coil diameter combined with theincrease in the number of fiber loops required would induce a seriousdegree of linear birefringence thus degrading the SOP of the light inthe sensing element with a concomitant reduction in measurementsensitivity and with increased environmental sensitivity.

Techniques to increase the sensitivity, and thus allow smaller andeasier methods to deploy sensors, have been investigated basedpredominantly on using materials which exhibit higher Verdet constants.Generally, these have been glass blocks made from flint or lead glass(see: T. Yoshino, "Optical fiber sensors for electric industry", SPIE,586, 30, 1985; and Y. Kuroda, Y. Abe, H. Kuwahara and K. Yoshinaga,"Field test of fiber-optic voltage and current sensors applied to gasinsulated substation", SPIE, 586, 30, 1985) with limited investigationof FR-5 glass (See: R. P. Tatam and D. A. Jackson, "Remote probeconfiguration for Faraday effect megnetometry", Optics commun. 72, 60,1989). FR-5 has a Verdet constant of ˜10⁻⁴ rad A⁻¹, typically 20 timesgreater than silica fiber. It is a glass doped with paramagnetic Nd ionsand therefore V(λ, T) follows a 1/T dependence; silica based opticalfiber and flint glass are diamagnetic materials and consequently thetemperature dependence of V(λ, T) Is negligible. This temperaturedependence of V(λ, T) for paramagnetic based sensing elements causessevere problems as changes in the recovered Faraday signal due totemperature changes are indistinguishable from changes in the electriccurrent/magnetic field, i.e. the scale factor is temperature dependent.For example, for an operating temperature range of -20° C. to +60° C.the Verdet constant for FR-5 changes by ˜30%. Fault protectionrequirements require ˜5% accuracy and metering 0.1 to 1% accuracy andtherefore temperature compensation is essential. Previous research hasconcentrated on signal processing techniques associated with recoveringthe Faraday signal and the techniques developed have significantlyimproved methods of deployment, allowing remote operation of sensingelements, as well as overcoming problems associated with `down-lead`sensitivity and operating point drift. However, the temperaturesensitivity was not addressed in these investigations. Severaltechniques for temperature compensation have been proposed based on thesensing element experiencing a permanent d.c. magnetic field component.The major disadvantage with these techniques is the requirement forpermanent magnets to be positioned at the sensing element, thusrequiring shielding from the very high magnetic fields present. Inaddition such techniques would be impractical with high Verdet constantoptical fiber since large diameter and long length magnets would berequired. Another proposed technique uses a bimetallic coil tomechanically rotate the sensing element thus increasing or decreasingthe effective magnetic field component acting on the sensing element.This technique requires the sensing element to be mounted in a complexmechanical assembly which is able to rotate over ˜30° and is thereforeunattractive for ease of installation and long term reliability.According to a first aspect of the present invention there is providedapparatus for sensing or measuring a magnetic field comprising

a light source for generating light having at least a component which ispolarized,

a sensing element arranged to transmit light from the source andcomprising material having a Verdet constant which depends on thetemperature of the material,

means for combining light from the source with light from the sensingelement to form an interference pattern,

means for providing a signal representative of the intensity of light socombined, and

means for deriving first and second signals dependent on the Verdetconstant of the material and the temperature thereof, respectively, fromthe signal representative of light intensity, whereby the intensity ofthe magnetic field at the element, substantially independent oftemperature, can be derived.

Preferably the means for combining is a Michelson interferometer and thesensing element is a high Verdet constant material in the form of aglass block or optical fiber, for example. The use of such materialsallows a higher sensitivity and smaller size sensing element. However,in some circumstances a specific material, such as a particular opticalfiber, may have certain advantages even though it has a relatively lowVerdet constant. Such materials may be used according to the inventionwhere the Verdet constant is temperature dependent.

The technique is completely passive and requires only the sensingelement to be placed in the measurement field; permanent magnets andmechanical compensation are not required.

The apparatus of the first aspect of the invention may include means forderiving a signal representative of the magnetic field at the elementfrom the first and second signals.

To simplify signal processing the apparatus may include means formodulating, with the said light intensity, a carrier signal such as canbe generated, for example, by cyclically varying either the frequency ofthe light or the length of one of the interferometer arms. Means fordemodulating the carrier signal may then be used to derive the firstsignal which varies according to the Faraday effect and temperature, andthe second signal which is dependent on temperature but not the Faradayeffect.

The sensing element of the invention is completely passive and opticallinks between the interferometer and both the light source and anoptical sensor, forming part of the means for sensing light intensityand change in phase, are preferably by way of a high birefringencemonomode fiber and a multimode fiber, respectively, allowing remotedeployment of the sensing element. Environmental perturbation of thesefiber links should not give rise to signal degradation. The small sizeand undistorted nature of its deployment, when not bent to form a loop,should result in significantly smaller susceptibility of the sensingelement to environmental perturbations, particularly vibration, thatgive rise to signal fading and scale factor changes.

According to a second aspect of the invention there is provided a methodof sensing or measuring magnetic field intensity comprising the steps of

generating light having at least one component which polarized,

transmitting the light generated through a sensing element in themagnetic field, the sensing element comprising material having a Verdetconstant which depends on the temperature of the material,

combining light from the source with light from the sensing element toform an interference pattern,

providing a signal representative of the intensity of light so combined,and

deriving first and second signals dependent on the Verdet constant ofthe material and the temperature thereof, respectively, from the signalrepresentative of light intensity, whereby the intensity of the magneticfield at the element, substantially independent of temperature, can bederived.

When polarized light is transmitted by a dielectric element in amagnetic field it experiences Faraday rotation. The Faraday effect isthe rotation of the azimuth, Φ_(F), of a plane of polarized light in thepresence of a magnetic field component, H, parallel to the direction ofpropagation of the light and is given by

    φ.sub.F =V(λ, T)  H.dL                          equation 1

where V (λ, T) is the material dependent Verdet constant, which is bothdispersive and temperature (T) dependent, λ is the wavelength of thelight and L is the interaction length.

If the dielectric element is used as a sensing element and forms part ofa Michelson interferometer illuminated by linearly polarized lightcoupled from the source to the interferometer using high birefringencefiber then conventional Michelson fringes are formed at the output. Thetransfer function for a Michelson interferometer may be written

    I∝(I.sub.1 +I.sub.2)(1+νcos ΔΦ)        equation 2,

where I is the observed intensity, I₁ and I₂ are the optical intensitiesin the two paths of the interferometer, ν is the visibility and ΔΦ isthe phase difference between the two recombining optical beams.

In equation 2 the visibility ν depends on I₁, I₂, the coherence lengthof the source and the relative state of polarization (SOP) of the tworecombining beams. From equation 1 the relative SOP depends on bothintensity of the magnetic field and temperature but phase difference,ΔΦ, that is the phase of the output fringes, depends only ontemperature. Temperature change in the sensing element alters theoptical path length, predominantly by means of a change in therefractive index, and hence varies ΔΦ. Thus a measurement of this phasechange gives direct information on temperature change occurring in thesensing element independent of the applied magnetic field. Thistemperature information is used to calculate the Verdet constant andallows the Faraday rotation to be used for magnetic field measurement ifcompensated for temperature changes of the sensing element.

Certain embodiments of the invention will now be described by way ofexample with reference to the accompanying drawings, in which:

FIG. 1 is a schematic diagram of an optical arrangement for apparatusaccording to the invention,

FIG. 2 is a schematic diagram of signal processing apparatus for usewith the arrangement of FIG. 1, and

FIG. 3 is a graph showing the relationship between temperature and atemperature dependent signal derived according to the invention.

FIG. 1 a coherent light source 10 projects linearly polarized lightthrough a lens 12 into a high-birefringence optical fiber 13 whichpreserves the polarization of the light. Light emerging from the fibre13 passes through another lens 14 to a beam splitter 15 having oneoutput beam which passes to a Faraday sensing element 16 and then by wayof a mirror 17 back through the Faraday sensing element 16 to the beamsplitter 15. The other beam from the splitter 15 passes through aquarter wave plate 18 to another mirror 19 where it is reflected backthrough the quarter wave plate to the beam splitter 15. The Faradaysensing beams from the element 6 and the quarter wave plate 18 arecombined by the beam splitter 15 and pass through a lens 21 into amultimode fiber 22 and then to an optical detector 23. It will beappreciated that the beam splitter 15, Faraday sensing element 16,mirror 17, quarter wave plate 18 and mirror 19 together constitute aMichelson interferometer.

The Faraday sensing element 16 employs a material which exhibits a highVerdet constant. Such elements may be glass blocks or fibers made fromflint or lead glass. A suitable sensing element is a rod made fromcommercially available FR-5 glass. Other possibilities for suitableblocks or fibers include TGG (terbium-gallium-garnet) which has a Verdetconstant of approximately twice that of FR-5. TGG is also availablecommercially. Other materials which may be used include YIG(yttrium-iron-garnet), BSO (bismuth-silicon-oxide), BIG (bismuthsubstituted iron-garnets) and RIG (rare earth substituted iron-garnets).

The quarter wave plate 18 is used to bias the interferometer for maximumsensitivity. Instead of a quarter wave plate 18, a linear analyzer couldbe utilized.

With the arrangement of FIG. 1 the output of the optical detector 23 isin accordance with equation 2 but a constant signal of this type isdifficult to process so a carrier signal is provided which is modulatedin amplitude by I and in phase by ΔΦ. A convenient method of applyingthis modulation is to use a laser diode as the light source 10 and tovary the frequency of the light by means of a sawtooth waveform obtainedfrom a signal generator 25 (FIG. 2) applied to the laser. Since thewavelength of the light from the light source 10 changes, the lengths ofthe arms of the interferometer, when measured in wavelengths, alsochange. Provided the lengths of the arms are slightly different, thesignal received by the optical detector 23 is amplitude modulated by theintensity I of equation 2 and its phase in relation to the output of thesignal generator 25 depends on the phase difference ΔΦ. Otherfrequencies are generated during the flyback period of the sawtoothwaveform and a bandpass filter 26 is used to select the modulatedfrequency. Filter 26 is connected to a lock-in amplifier 27 which alsoreceives a reference signal from the signal generator 25. The amplifier27 acts as a demodulator to provide a signal proportional to theamplitude of the signal from the filter 26, this amplitude beingproportional to I of equation 2 and therefore proportional to magneticfield intensity surrounding the Faraday sensing element 16 andtemperature. The lock-in amplifier 27 also acts as a phase detectorproviding a signal proportional to the difference in phase between itstwo input signals and therefore proportional to the temperature ofFaraday sensing element 16. These amplitude and phase signals areapplied to a microcomputer 28 programmed to compensate the signalproportional to amplitude for temperature variations and to provide anoutput representative of the magnetic field surrounding the Faradaysensing element 16. The computer stores a previously obtainedcalibration of the Verdet constant in relation to temperature for thematerial of the Faraday sensing element 16. Having derived thetemperature, It uses the calibration to derive the Verdet constant andthen magnetic field intensity from the Verdet constant. The calibrationmay be in the form of a look-up table or an equation. Thus a signalrepresentative of temperature may also be provided. Clearly when themagnetic field depends on an electric current, the field signal isproportional to current.

An alternative way of modulating the signal at the optical detector 23is to mount the mirror 19 on a piezoelectric cylinder so that itsposition in relation to the quarter wave plate 18 can be varied byapplying a sawtooth waveform to the cylinder. With such an arrangementthe same method of signal processing can be used but it is not necessaryfor the two arms of the interferometer to be of different lengths. Inboth these arrangements the optimum operating point Is obtained from theinterferometer by biasing It using the quarter wave plate 18 in one armsuch that the azimuth of the recombining beams are at 45° with respectto each other. Subsequent azimuth modulation of one of the linear statesgives rise to maximum rate of change of visibility of the fringes. Thequarter wave plate can be replaced by a linear analyzer, that is apolaroid element orientated such that it rotates polarization by 45°.

A sinusoidal variation in light frequency or mirror position may be usedas an alternative to the sawtooth waveform but other arrangements forsignal processing are then required.

For higher frequency applications than can be obtained with a laserdiode, a Bragg cell can be positioned between a laser providing constantfrequency light and the beam splitter. The Bragg cell also receives asignal from the signal generator 25 and produces two light beams, one atthe frequency of the laser light and one at the same frequency plus orminus the frequency of the signal from the signal generator 25. Thedifference of frequency between the two beams is modulated by theinterferometer and the output received at the optical detector 23 isprocessed in the same way.

The sawtooth frequency or mirror position variation techniques arepseudo-heterodyne arrangements, the Bragg cell technique is a trueheterodyne arrangement and an active homodyne alternative is nowmentioned. Variations in the output of the optical detector 23 may beused as an error signal in a servo system which changes the frequency ofthe light from the light source 10 to tend to reduce the variations. Ifthe servo system has a low bandwidth, the control signal of the servorepresents temperature change while the output from the optical detector23 represents magnetic field intensity. With a wide bandwidth servo, thecontrol signal contains both "magnetic field" signals and "temperature"signals which can be separated by virtue, usually, of separate ranges offrequency variation.

Results obtained for temperature sensing with the arrangement of FIGS. 1and 2 are shown in FIG. 3. For these results the mirror 19 was mountedon a piezoelectric cylinder driven by a voltage from the signalgenerator 25. This enabled pseudo-heterodyne signal processing to beimplemented. A solenoid coil (not shown) was placed around the Faradaysensing element 16 to allow test fields to be applied. FIG. 3 shows theelectric current, measured from the visibility modulation, andtemperature plotted against time. Initially the current to the coil waschanged whilst holding the temperature constant. The temperature wasthen increased by ˜60° C. and the measured current decreased whileapplied current was kept constant. Compensation for the temperaturedependence of V(λ,T) is shown by the dashed line 30.

Since cosΔΦ in equation 2 repeats every 2π the temperature indicationmentioned above is ambiguous. Some ways in which this ambiguity can beresolved will now be outlined. If the interferometer is Illuminatedusing a broad band, or "white light" source, fringes are only observablewhen the optical path length differences between the two recombiningbeams are almost identical, that is within the coherence length of thesource which is typically a few microns. In the system described abovetemperature changes in the sensing element 16 change the relativeoptical path lengths and the technique of a remote balancinginterferometer can be used to recover the temperature change butamplitude modulation of the visibility is still used to recover themagnetic field signal as described above. In a remote balancinginterferometer a signal dependent on the intensity of the fringes isderived and is used to maintain maximum fringe intensity by movement ofa mirror equivalent to one of the mirrors 17 and 19. The signal used fordriving this mirror is representative of temperature and may be used forthis purpose, and for compensating the intensity signal. The balancinginterferometer is described by R. Jones, M. Hazell and R. Welford in "AGeneric Sensing System Based on Short Coherence Interferometer", SPIE1314. 315, 1990. Other possible techniques which may possibly be appliedin resolving the temperature ambiguity are described in two furtherpapers: firstly "Dual Wavelength Approach to Interferometric Sensing" byA. D. Kersey and A. Dandridge, SPIE 7898. 176, 1987; and "PartiallyCoherent Sources in Interferometric Sensors" by S. A. Al-Chalabi, B.Culshaw and D. E. N. Davis, Proc. First. Int. Conf. on Optical FibreSensors, IEE Conference Publication 221,132, London 1983.

The primary use of the invention is expected to be for currentmeasurement and the sensing element 16 then has to be positioned in themagnetic field due to the current. The sensing element 16 may simplyform one or more turns round a conductor carrying the current or theelement in the form of an optical fiber may be in the form of one ormore turns around the conductor. Another possibility is to construct aclosed optical circuit using the sensing element 16 and thereforeutilize the advantages offered from Ampere's law, that is insensitivityto conductor position within the optical circuit and immunity to fieldsfrom surrounding conductors. The optical circuit can be made from glassblocks or rods forming the sides of a hollow rectangle with theconductor passing through the rectangle. Light from the beam splitterenters one rod or block axially at a corner of the rectangle and isreflected at the corners of the rectangle by mirrors or a diagonalinterface so that it follows the sides until it reaches the entry cornerwhere it is reflected by a mirror back along its incoming path. In analternative arrangement the sensing element 16 may be only a fewcentimeters long and positioned in the air gap of a magnetic coresurrounding a conductor.

It will be apparent that the invention can be put into effect in manyother ways than those specifically described. For example other methodsof providing a modulated carrier signal, or equivalent, may be used, asmay other methods for resolving the temperature measurement ambiguity.Other interferometers such as the Fabry-Perot or Mach-Zehnderinterferometer, or modifications thereof, may be used instead of theMichelson interferometer. If a Fabry-Perot interferometer were used,this would involve alteration of the optical arrangement shown in FIG. 1as follows. The mirror 19 would be replaced by a partially reflectingsurface on that end of the Faraday sensing element 16 closest to thebeam splitter 15. The quarter wave plate 18 would be moved from itsposition shown in FIG. 1 and placed between the sensing element 16 andthe mirror 17. In operation, light would recombine from reflections atthe partially reflecting surface and at the mirror 17. Theinterferometer is thus configured as a remote low finesse Fabry-Perotinterferometer. In practice, the interferometer might be made as anintegral unit, in which case the quarter wave plate 18 would be glued tothe sensing element 16, the mirror 17 would take the form of areflective surface applied to the quarter wave plate, and this wholeassembly would be glued to the beam splitter 15 via the partiallyreflecting surface.

Light from the source may be elliptically polarized when the abovedescribed methods may be used, with some modification. As anotheralternative light from the source may be circularly polarized when, itcan be shown, the phase change in the sensing element is dependent onboth the Verdet constant and temperature but not the amplitude of theintensity. In general the phase or frequency change due to the Verdetconstant is different from that due to temperature so signalsrepresentative of the former can usually be separated from thoserepresentative of the latter allowing correction of the Verdet constantfor temperature.

I claim:
 1. Apparatus for sensing or measuring a magnetic fieldcomprising:a light source for generating light having at least acomponent which is polarized; a sensing element arranged to transmitlight from the light source and comprising material having a Verdetconstant which depends on a temperature of the material; means forcombining light from the light source with light from the sensingelement to form an interference pattern; means for providing a signalrepresentative of an intensity of light combined by the means forcombining light; and means for deriving first and second signalsdependent on the Verdet constant of the material and the temperaturethereof, respectively, from the signal representative of the intensityof light; whereby an intensity of a magnetic field at the sensingelement, substantially independent of temperature, can be derived. 2.Apparatus according to claim 1, wherein the means for deriving the firstand second signals comprises:means for deriving the first signal as asignal representative of an amplitude of fringes of the interferencepattern; and means for deriving the second signal as a signalrepresentative of a change in phase of polarized light from the lightsource due to transmission through the sensing element.
 3. Apparatusaccording to claim 1 or 2, wherein the means for combining comprises aMichelson interferometer and light from the light source issubstantially all linearly polarized.
 4. Apparatus according to claim 1,including means for deriving a signal representative of the magneticfield at the sensing element from the first and second signals. 5.Apparatus for sensing or measuring a magnetic field comprising:a lightsource for generating light having at least a component which ispolarized; a sensing element arranged to transmit light from the lightsource and comprising material having a Verdet constant which depends ona temperature of the material; means for combining light from the lightsource with light from the sensing element to form an interferencepattern; means for providing a signal representative of an intensity oflight combined by the means for combining light; means for derivingfirst and second signals dependent on the Verdet constant of thematerial and the temperature thereof, respectively, from the signalrepresentative of the intensity of light; means for modulating a carriersignal with said intensity of light; and demodulating means foramplitude demodulating the carrier signal to derive said first signaland for phase demodulating the carrier signal to derive said secondsignal; whereby an intensity of a magnetic field at the sensing element,substantially independent of temperature, can be derived.
 6. Apparatusaccording to claim 5, wherein the means for modulating comprises:meansfor generating a cyclically varying signal; and means for varying afrequency of the light source according to the cyclically varyingsignal; the demodulating means being coupled to receive the cyclicallyvarying signal as a reference signal for phase demodulating the carriersignal.
 7. Apparatus according to claim 5, wherein:the means formodulating comprises:means for generating a cyclically varying signal;and a Bragg cell interposed between the light source and the means forcombining light, with the means for generating the cyclically varyingsignal coupled thereto; the demodulating means being coupled to receivethe cyclically varying signal as a reference signal for phasedemodulating the carrier signal.
 8. Apparatus according to claim 1,wherein said material of the sensing element has a high Verdet constant.9. Apparatus according to claim 1, wherein said material is chosen froma group consisting of:FR-5 glass, terbium-gallium-garnet,yttrium-iron-garnet, bismuth-silicon-oxide, bismuth substitutediron-garnets, and rare earth substituted iron-garnets.
 10. Apparatusaccording to claim 1 or claim 8 or 9, wherein:the light source generatescircularly polarized light; and the means for deriving first and secondsignals comprises:means for deriving the first signal as a signaldependent on a change of phase of light from the light source whichoccurs on transmission through the sensing element due to the Verdetconstant of said material, and means for deriving the second signal as asignal dependent on a change of phase of light from the light sourcewhich occurs on transmission through the sensing element due to thetemperature of said material.
 11. Apparatus for sensing or measuringelectric current comprising apparatus according to claim 1, wherein thesensing element is arranged, adapted or positioned to be subject to amagnetic field due to an electric current which is to be sensed ormeasured.
 12. Apparatus according to claim 11, wherein the sensingelement is such that, in operation, it can be used in one of thefollowing configurations:with a conductor carrying said current arrangedas one or more turns around the sensing element; with the sensingelement arranged as one or more turns around a conductor carrying saidcurrent; and with the sensing element positioned in a gap in a magneticcore which surrounds a conductor carrying said current.
 13. Apparatusaccording to claim 11, wherein the sensing element is arranged as anoptical circuit comprising a solid portion of said material surroundinga conductor carrying said current, the solid portion of said materialforming a polygon having a plane at an angle to the conductor. 14.Apparatus according to claim 1, including means for at least partiallyresolving any ambiguity in the second signal in relation to anindication of temperature thereby.
 15. Apparatus according to claim 14,wherein:the means for combining comprises a Michelson interferometer;the light source is relatively broad band and the Michelsoninterferometer is a balancing interferometer in which a control signalis developed which is used to maximize an intensity of fringes of theinterference pattern generated by the Michelson interferometer; and themeans for resolving error employs the control signal to provide, atleast partially, an indication of the temperature of the sensingelement.
 16. Apparatus according to claim 2 or claim 8 or 9, wherein:thelight source is relatively broad band; and the means for combiningcomprises a balancing interferometer in which a control signal isdeveloped which is used to maximize the intensity of the fringes of theinterference pattern generated by the interferometer; and the means forderiving the second signal includes means for generating the controlsignal, the control signal being representative of said change in phase.17. A method of sensing or measuring magnetic field intensity comprisingthe steps of:generating light having at least one component which ispolarized; transmitting the generated light through a sensing element inthe magnetic field, the sensing element comprising material having aVerdet constant which depends on a temperature of the material;combining the generated light with light from the sensing element toform an interference pattern; providing a signal representative of theintensity of light so combined; and deriving first and second signalsdependent on the Verdet constant of the material and the temperaturethereof, respectively, from the signal representative of the intensityof light, whereby the intensity of the magnetic field at the sensingelement, substantially independent of temperature, can be derived.
 18. Amethod according to claim 17, including a step of deriving a signalrepresentative of the magnetic field at the sensing element from thefirst and second signals.
 19. A method of sensing or measuring magneticfield intensity, comprising steps of:generating light having at leastone component which is polarized; transmitting the generated lightthrough a sensing element in the magnetic field, the sensing elementcomprising material having a Verdet constant which depends on atemperature of the material; combining the generated light with lightfrom the sensing element to form an interference pattern; providing asignal representative of the intensity of light so combined; andderiving first and second signals dependent on the Verdet constant ofthe material and the temperature thereof, respectively, from the signalrepresentative of the intensity of light, whereby the intensity of themagnetic field at the sensing element, substantially independent oftemperature, can be derived; deriving a signal representative of themagnetic field at the sensing element from the first and second signals;modulating a carrier signal with said intensity of light; amplitudedemodulating the carrier signal to derive said first signal; and phasedemodulating the carrier signal to derive said second signal.
 20. Amethod for sensing or measuring electric current using the method of anyone of claims 17 to 18, wherein the sensing element is subject to amagnetic field due to an electric current which is to be sensed ormeasured.
 21. Apparatus for measuring temperature comprising apparatusaccording to claim 1, further comprising means for indicating thetemperature of the material in dependence on the second signal. 22.Apparatus for sensing or measuring a magnetic field comprising:a lightsource for generating light having at least a component which ispolarized, light from the light source being substantially all linearlypolarized; a sensing element arranged to transmit light from the lightsource and comprising material having a Verdet constant which depends ona temperature of the material; means for combining light from the lightsource with light from the sensing element to form an interferencepattern, wherein the means for combining comprises a Michelsoninterferometer; means for providing a signal representative of anintensity of light combined by the means for combining light; means forderiving first and second signals dependent on the Verdet constant ofthe material and the temperature thereof, respectively, from the signalrepresentative of the intensity of light; means for deriving a signalrepresentative of the magnetic field at the sensing element from thefirst and second signals; means for modulating a carrier signal withsaid intensity of light, said means for modulating comprising:means forgenerating a cyclically varying signal, and means for varying a lengthof one arm of the Michelson interferometer according to the cyclicallyvarying signal; means for amplitude demodulating the carrier signal toderive said first signal and for phase demodulating the carrier signalto derive said second signal, the demodulating means being coupled toreceive the cyclically varying signal as a reference signal for phasedemodulating the carrier signal; whereby an intensity of a magneticfield at the sensing element, substantially independent of temperature,can be derived.
 23. A method of sensing or measuring magnetic fieldintensity, comprising steps of:generating light having at least onecomponent which is polarized; transmitting the generated light through asensing element in the magnetic field, the sensing element comprisingmaterial having a Verdet constant which depends on a temperature of thematerial; subjecting the sensing element to a magnetic field due to anelectric current which is to be sensed or measured; combining thegenerated light with light from the sensing element to form aninterference pattern; providing a signal representative of the intensityof light so combined; and deriving first and second signals dependent onthe Verdet constant of the material and the temperature thereof,respectively, from the signal representative of the intensity of light,whereby the intensity of the magnetic field at the sensing element,substantially independent of temperature, can be derived; deriving asignal representative of the magnetic field at the sensing element fromthe first and second signals; modulating a carrier signal with saidintensity of light; amplitude demodulating the carrier signal to derivesaid first signal; and phase demodulating the carrier signal to derivesaid second signal.